Self-assembled monolayers (SAMs) have attracted much attention in areas such as device engineering because of the versatility they provide for surface modification. SAMs are highly ordered molecular assemblies that form spontaneously by chemisorption of functionalised molecules on a variety of substrates such as metals, Indium Tin Oxide (ITO), silicon, and glass. These molecules organise themselves laterally, most commonly via van der Waals interactions between long aliphatic chains. The principles and practice of deposition of monolayers are described in detail in a publication by A Ulman entitled “Introduction to Thin Organic Films: From Langmuir-Blodgett to Self-Assembly”, published by Boston Academic Press, 1991. SAMs have found widespread research interest because of potential applications related to control over wettability, biocompatibility and corrosion resistance of surfaces.
For many electronic, optical and electro-optical devices for example, the ability to modify the properties of surface areas of the devices makes SAMs attractive for many applications, such as modification of surface hydrophobicity, packaging and electrical insulation. Furthermore, as SAMs exhibit excellent barrier properties, they are considered very appropriate for use as protective coatings on metal surfaces because they form thin highly crystalline barrier films. Gold has found widespread application and, for example, is used extensively in the electronics industry in integrated circuit technology. Also, as a relatively inert metal it has also been used as a protective layer in certain chemical environments, such as a liner material for the ink chambers in ink jet print heads. However, gold will dissolve under appropriate chemical or electrochemical conditions, so the ability of SAMs to provide a very thin protective layer to such metal layers in harsh chemical environments where metal layer corrosion is known to occur is also considered extremely attractive. However, SAMs have been found to exhibit certain drawbacks which, to date, have severely limited their commercial application in industrial processes.
To date, the SAM material is deposited by dissolving the material in an appropriate solvent and, as such, the monolayer formation over the required flat surface areas, which usually include surface discontinuities arising from design features dictated by the practical application of devices, is difficult to control. As the layers are self-aligning, they often exhibit molecular sized defects or holes in the layer. These defects can limit their use as barrier or passivation layers in certain industrial applications because the barrier properties provided by the densely packed molecules of the SAM material can be breached through the molecular sized defects.
Furthermore, although SAMs are typically in the order of only about 2 nm thickness, they are relatively slow to deposit. Typical deposition times range from several hours to a few days with the normal solvents used for the compounds. Heavily fluorinated compounds, such as thiols carrying aliphatic tails with multiple fluorine substituents, have been used to form hydrophobic SAMs on gold substrates. A measure for the hydrophobicity of a surface is the contact angle between a drop of water and that surface. These SAMs are quite robust and are stable up to 150° C. as described by Fukushima et al in The Journal of Physical Chemistry, B, (2000) 104, pages 7417 to 7423, so it can be appreciated that such monolayers could find widespread industrial application if the concerns associated with fabrication can be met.
Typically, SAM molecules comprise a head that is attracted to and bonds with the substrate material and a functionalised tail, such as an alkyl tail. Such single chain, linear perfluoroalkyl SAMs have been fabricated on metallic, glass and silicon type substrates. For metallic substrates, the head preferably comprises a thiol and for glass and silicon type substrates the head preferably comprises a silane.
In particular, for metallic substrates, which may consist of a layer of Au, Ag, Cu, Pd, Fe, Hg, GaAs, ITO, or Fe2O3 on a suitable supporting medium, the SAM may typically comprise a substance including semi-fluorinated, sulphur-containing compounds of the formula:
wherein X can be (but is not necessarily limited to) R—SH, RS—SR, or R—S—R (where R denotes the rest of the molecule). Preferably, X is a thiol.
The numbers m and n denote the number of fluorinated and non-fluorinated carbon atoms, respectively, and lie within the range of 1-20. Y preferably indicates a CF3 functional group. Y may be further modified to incorporate one or more substituents such as vinyl, styryl, acryloyl, methacryloyl or alkyne for further functionalisation or cross-linking, with one or more spacer group such as CH2 to facilitate attachment.
Self-assembled monolayers on glass, mica, SiO2, Al2O3, or Ga2O3 typically involve semi-fluorinated silane derivatives of the formula:
wherein Si can be (but is not necessarily limited to) SiCl3, Si(OCH3)3, Si(OCH2CH2CH3)3, Si(OCH3)2Cl, or Si(CH2CH3)2Cl. Preferably, Si is SiCl3. The numbers m and n denote the number of fluorinated and non-fluorinated carbon atoms, respectively, and lie within the range of 1-20. Y preferably indicates a CF3 functional group. Y may be further modified to incorporate one or more substituents such as vinyl, styryl, acryloyl, methacryloyl or alkyne for further functionalisation or cross-linking, with one or more spacer group such CH2 to facilitate attachment.
Compressed carbon dioxide (CO2) is known to be a clean and versatile solvent medium for a wide range of materials, including heavily fluorinated compounds. Supercritical CO2 has been used for polymer synthesis and polymer processing. Such use is described in an article by A Cooper entitled “Polymer Synthesis and Processing using Supercritical Carbon Dioxide”, published in The Journal of Materials Chemistry, 2000, 10, pages 207 to 234. A supercritical fluid may be defined as a substance for which both temperature and pressure are above the critical values for the substance and which has a density close to or higher than its critical density. For CO2 the critical density is recognised to be 0.47 g cm−3, and the critical temperature and pressure are recognised to be 31.1° C. and 73.8 bar. Compressed CO2 has also been proposed as a solvent for the preparation of organic molecules, as described in a Special Issue of Chemical Review, 1999, 99 Volume 2.
Dendrimers are a type of regular-branched polymeric molecule. Their unusual structures can be precisely controlled at the molecular level and they have unique properties. In particular, they are spherical, have a single molecular weight and can be tailored to provide desired functions. A schematic view of a dendrimer 1 is shown in FIG. 1.
Dendrons are also regular-branched polymeric molecules and their structures can also be precisely controlled. However, they are wedge-shaped rather than spherical and comprise a focal point from which the branches originate. A schematic view of three dendrons 2 attached to a surface 3 is shown in FIG. 2. As shown in the figure, one chain extends from the focal point 4. The chain has two branches 5, 6 extending from it. Two further branches extend from each of the branches 5, 6 and so on. Each fork in a branch may be considered as the start of a different “layer” in the dendron. Thus, the dendrons 2 shown in FIG. 2 have five layers referenced 4, 5, 7, 8 and 9 respectively. Different dendrons may have different numbers of branches extending from each branch and different numbers of layers.
Dendron thiols, that is dendrons with a thiol group at the focal point are also known and have been considered as building blocks in nanotechnology.